1. Field of the Invention
The present invention relates to an optical waveguide device and a method of manufacturing such an optical waveguide device, and more particularly to an optical waveguide device comprising an optical waveguide mounted on an LiNbO.sub.3 substrate by diffusing Ti therein, i.e., a Ti-diffused LiNbO.sub.3 waveguide, and a method of manufacturing such an optical waveguide device.
2. Description of the Related Art
Optical waveguides are generally capable of confining a radiation within a certain region and guiding and propagating the confined energy of the radiation parallel to the axis of the waveguide passage. At present, efforts are being made to replace optical light guides, typically, optical fibers, with optical waveguides for thereby reducing the size of optical components.
The optical waveguides include semiconductor waveguides of GaAs and InP, dielectric (glass) waveguides using glass substrates, and ferroelectric crystal waveguides of LiNbO.sub.3 and LiTaO.sub.3 crystals.
In particular, optical devices such as optical-waveguide modulators for imparting information through electrodes to a light beam that is transmitted through optical waveguides comprise Ti-diffused LiNbO.sub.3 waveguides in which Ti is diffused in an LiNbO.sub.3 crystal that has excellent electrooptical properties.
As shown in FIG. 13 of the accompanying drawings, a Ti-diffused LiNbO.sub.3 waveguide comprises an LiNbO.sub.3 substrate 100 and a metal film 102 of Ti deposited to a thickness of several hundreds .ANG. on the LiNbO.sub.3 substrate 100 and thermally diffused at a temperature of about 1000.degree. C. for 4-10 hours.
As shown in FIG. 14 of the accompanying drawings, the Ti-diffused LiNbO.sub.3 waveguide which has a small smooth step 104 on the Ti-diffused region of the LiNbO.sub.3 substrate 100 has been considered as being characteristically good. Those Ti-diffused LiNbO.sub.3 waveguides which have surface irregularities (surface roughness) on the step 104 have been considered as being not preferable because those surface irregularities tend to increase a light beam propagation loss.
In order to prevent the surface roughness, it has heretofore been customary to introduce water vapor while Ti is being diffused or place a powder containing Li, such as a powder of LiNbo.sub.3 in the diffusion chamber.
The loss of an optical waveguide is determined by a combination of the propagation loss and a coupling loss with respect to the optical mode of an optical fiber. The coupling loss is represented by the ratio of the mode shape of a light beam propagated through the optical waveguide, i.e., the shape of a light beam with respect to a plane perpendicular to the optical axis of the optical waveguide, to the mode shape of a light beam propagated through the optical fiber, i.e., the shape of a light beam with respect to a plane perpendicular to the optical axis of the optical fiber. The coupling loss is larger as the ratio is larger or smaller than 1.
As shown in FIG. 14, on the optical waveguide having a smooth surface configuration for the LiNbO.sub.3 substrate 100, the step 104 of the Ti-diffused region has a uniform height distribution, and the shape of the plane perpendicular to the optical axis with respect to the diffused distribution of Ti, i.e., the mode shape BS of the light beam propagated through the optical waveguide, is substantially circular. This is considered to result from substantially isotropic diffusion of Ti due to the smooth surface of the step 104.
The coupling loss of the optical fiber with respect to a general optical fiber which propagates light with a randomly varying plane of vibration (plane of polarization) is small. However, the coupling loss of the optical fiber with respect to an optical fiber in which the mode shape of a light beam is not circular, among polarization-maintaining fibers which propagate light with a plane of polarization being kept in a certain direction, is large. Particularly, the coupling loss of the optical fiber with respect to an optical fiber in which the aspect ratio of the mode shape of a light beam is not 1, e.g., an optical fiber in which the core is of an elliptical cross-sectional shape, is very large.
If an optical component is to be fabricated using a polarization-maintaining fiber, then since the coupling loss described above greatly affects the optical characteristics (device characteristics) of the optical component, it is necessary to reduce the coupling loss of an optical waveguide even in the sacrifice of a certain propagation loss.